1. Field of the Invention
The present invention relates to a method for carrying out partial etching or pattern etching through the use of an electrolytic reaction and to a method for producing a photovoltaic element and a method for treating a defect of a photovoltaic element, using the foregoing method.
2. Related Background Art
Hitherto, as a method for etching a metal or a transparent conductive film, there has been employed the chemical etching method such as disclosed in, for example, Japanese Patent Application Laid-Open No. 55-108779, U.S. Pat. No. 4,419,530, and so on. This is a method for first forming a positive pattern by silk screen printing or by a photoresist and thereafter treating the metal or the transparent conductive film corresponding to negative portions (exposed portions) with an etchant such as ferric chloride solution or nitric acid.
However, the chemical etching method necessitates such treatments as application of a resist onto an object to be etched (simply referred to as xe2x80x9cetching objectxe2x80x9d) and removal of the resist, which complicates the steps and increases the operation cost. In addition, the chemical etching method required long operation time for some types of etching objects because of its low reaction rate. Further, the chemical etching method did not permit a non-contact process and thus had another problem that the yield was low.
On the other hand, for carrying out the partial etching or patterning by use of an electrolytic reaction, the electrolytic etching was conducted after a pretreatment of application of a resist to an etching object, followed by exposure and hardening to obtain a pattern. In this case, the resist used needs to be removed after the etching. For example, Japanese Patent Application Laid-Open No. 62-290900 discloses a method for making a resist pattern in close contact with an electroconductive film, immersing it in an aqueous solution of hydrochloric acid, supplying an electric power to pattern the film in the other portions than the resist, and removing the resist, thereby completing the etching. Specifically, it discloses using flat plate electrodes as counter electrodes and performing an electrolytic treatment by a power source in an electrolyte to form a desired pattern in the etching object.
However, although this method permits the non-contact process during electrolysis, it also requires the pretreatment of application of the resist and the posttreatment of removal of the resist as the chemical etching does. Therefore, this method also has the problems of the impracticability of perfect non-contact process in the pretreatment and in the posttreatment, the incapability of curtailing the operation cost, the large number of steps, and lowering in the yield. It has another problem that if bubbles formed during electrolysis stay on the resist pattern, patterning can not be carried out precisely.
Further, as a method free from the pretreatment of the resist pattern on the etching object, Japanese Patent Application Laid-Open No. 5-93300 discloses a method for putting a probe electrode into a capillary having an aperture at the tip, placing a small gap between the peripheral surface of this tip aperture and a solid surface of a work as a working electrode, and carrying out electrolysis while allowing the electrolyte to flow through the tip aperture.
However, although this method permits the non-contact patterning process of the etching object without the pretreatment, it has a problem of difficult pattern control due to spread of the solution in the in-plane direction because the method is a pinpoint etching technique. Further, this method has another problem of rounding of edge portions and longer operation time when applied to patterning in a wide area. In addition, the amount of gas generated during electrolysis becomes large depending upon the kind of the etching object or the electrolyte, and the gas generated in the early stage of reaction stays in the form of bubbles between the etching object and the electrode. Therefore, since the electrolytic reaction is inhibited in the portions where the bubbles stay, there arises a problem that disconnection or chipping of a patterning line occurs.
An object of the present invention is to provide an electrolytic etching method capable of solving the problems of the prior art and etching an etching object in a non-contact manner, at low cost, in the reduced number of steps, in reduced operation time, and with enhanced patterning accuracy.
The inventors have extensively and intensively studied the etching methods utilizing reduction, oxidation, and dissolution phenomena by an electrochemical reaction of a metal or a transparent conductive film, found out conditions for enhancing the etching accuracy, and accomplished the present invention by further detailed analysis of the knowledge obtained by the inventors on the etching method permitting the stable treatment over a long period without such an adverse effect as peeling off.
The gist of the invention is that the etching treatment is carried out by an electrochemical reaction between the etching object and the etching electrode and the contact angle of the electrolyte to the etching object is not more than 70xc2x0 during execution of patterning, whereby influence of the bubbles generated during electrolysis can be controlled. As a result, the problem of defective etching including the disconnection and chipping of patterning line is overcome. Further, the present method can also be applied to fabrication of photovoltaic elements, so that photovoltaic elements can be fabricated with good characteristics, in a high yield, and with high reliability.
The present invention has been accomplished based on these findings, and the operation of the present invention will be described below.
The surface tension of the electrolyte is lowered and the contact angle of the electrolyte to the etching object is adjusted to not more than 70xc2x0, preferably to 10xc2x0-70xc2x0, whereby it is possible to prevent the phenomenon that the gas is generated in a large volume by the electrolytic reaction and forms a large bubble, which stays, and whereby it is also possible to prevent the phenomenon that the bubbles instead become so small as to be easy to cohere and a cohering body of the bubbles stays.
When the electrochemical reaction is allowed to take place between the etching object and the etching electrode, lowering the surface tension of the electrolyte can break or inhibit the bubbles of the generated gas, whereby it is possible to control staying (or residence) of a large bubble somewhere between the etching object and the electrode. As a result, it becomes possible to effectively carry out the electrolytic etching.
When an additive is added in an amount of 0.01%-10% of the total weight of the electrolyte, excellent patterning by electrolytic etching can be performed without hindering the electrolytic reaction while controlling the residence of bubbles.
By adding to the electrolyte at least one selected from polyethylene glycol, polypropylene glycol, acetylenic alcohol, ethanol, and copolymers thereof, and surfactants, as a substance for lowering the surface tension, the surface tension can effectively be lowered. As a result, the residence of bubbles can be inhibited.
By combining the electrolytic etching treatment with patterning processing, the processing can be performed at a high treating rate without being affected by the temperature of the treating solution or the like.
Combining the electrolytic etching with mask etching enables patterning including a curved line or the like.
Forming the etching electrode in a desired etching pattern enables non-contact patterning.
When the electrolyte contains a Lewis acid or a Lewis base, they serve as a carrier of charge by the electron-pair transfer, so that the electrolytic reaction can be promoted without generating many hydrogen ions, whereby generation of the bubbles posing the problem upon etching can be reduced.
When an aqueous solution is used as the electrolyte, the electrolyte is less volatile and thus the sequential steps can be performed readily with little affection by vaporization or the like.
Allowing a direct current, a pulse current, or an alternating current to flow between the etching electrode and the etching object becomes it possible to control the line width and the treatment time.
By effecting washing with water and thermal treatment of the etching object after the etching treatment, the additive is prevented from reacting with the etching object, so that the adverse effect such as peeling off will not occur.
By using as the etching object a substrate on which a transparent conductive film is deposited, the electrolytic etching of the present invention can be applied to fabrication of various devices including solar cells, photosensitive devices, light-emitting devices, liquid-crystal displays, and so on.
By using as the etching object a substrate for a photovoltaic element having at least one of a metal layer and a transparent conductive film thereon, precise patterning is achieved with accuracy, whereby a photovoltaic element can be obtained with excellent appearance, characteristics, and reliability.
By using as the etching object a photovoltaic element having a semiconductor layer provided on a substrate and an electroconductive layer provided on the semiconductor layer, etching can be achieved with high accuracy, so that a photovoltaic element can be produced with high performance.
By reducing a conductive layer in the periphery of a short-circuit path by the electrolytic etching method of the present invention, the conductive layer on the short-circuit path can be selectively etched, whereby the performance of the photovoltaic element can be restored.